Green water as shown in the water feature above
is because the sunlight has made micro organisms (algae) in the
water turn green. It is nothing to do with the water turn over rate. (Its
a popular misconception that moving water does not turn green.)

There are several ways around it, pond owners use a U.V
clarifier and filter. Commercial fountains use chemical dosing or silver ion units.
along with sand filters

Each system has its own "problems" Most domestic
fountains do not have room for a filter and U.V. and commercial products*
are too expensive so green water it is then.

There is a slight exception to this, if the feature is
in a sump, and the sump is covered so that the water is not exposed to
sunlight for long, the water will still turn green, but it will take
considerably longer to do it as there is not a lot of water exposed to the
sun.

For more information regarding fountain hygiene you
may want to see our page here